performance optimization of cooling towerindianpowerstations.org/presentations made at...

CenPEEP

Performance Optimization

of

Cooling Tower

Surendra Prasad, DGM (CenPEEP)

Partha Nag, DGM (CenPEEP)

Manoj Jha, Manager (CenPEEP)

International O&M Conference – IPS 2012

13th to 14th February, 2012

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Used to cool the condenser return by the interaction of Air & Water through fills.

Two Types:

Natural Draft: Circulation due to density difference.

High capital cost but low operational cost.

Induced Draft:

Forced Circulation by Fan. Lesser capital

cost but high operational cost.

Two Types of Fills:

Splash Fills: Splashing of Water into small droplets thereby increasing the Surface Area of

interaction.

Film Fills:

Increases the time of interaction between Air

& Water to facilitate cooling.

Cooling Tower - Types

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For a 200 MW Unit : Cooling Tower Heat Duty is equivalent to approx. 275 MW

For a 500 MW Unit : Cooling Tower Heat Duty is equivalent to approx. 700 MW

Importance of Cooling Tower Performance Monitoring

Total No. of installed Cooling Tower : 102

Cross flow splash bar fill type - 28

Counter flow splash bar fill type - 18

Counter flow film fill type - 46

Natural Draught Splash type/film Fill - 10

Types of Cooling Tower in NTPC

Cooling Tower Performance

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Importance of Tower Capability over Effectiveness

Effectiveness Capability

► Effectiveness is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature.

► Effectiveness is the function of Range and Approach only

► Effectiveness = Range / (Range + Approach).

► Missing Factors:

• Inlet air WBT

• CW Flow

• Wind velocity

• Fan Power

Effectiveness is used where ever accurate CW Flow measurement is a constraint.

► Capability is as per CTI ATC – 105 code.

► Capability is defined as the percentage of water that the tower can cool to the design cold water temperature when the parameters are all at their design value.

► Capability is function of inlet air WBT, Range, CW flow, fan power, wind velocity.

► Capability = Measured flow x { Design KW of fans}0.333

Predicted CW flow x {Test KW of Fans}0.333

► Predicted CW flow is calculated from Manufacturer curves.

► CW Flow measurement : 3-hole pitot tube / Ultrasonic flow meter

► Multiple functional variables of independent nature.

► Comparability of CT Performance

► Single Cell CT Capability as complementary measurement for Performance Evaluation

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Determination of CT Performance

Acceptance Test Code for Water- Cooling Towers: CTI ATC - 105.

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Cold water temperature expected to improve by 2-3 deg C by improving Tower performance

This will improve Condenser Vacuum by 6 – 9 mm Hg

Improvement in Heat Rate 12 – 18 kcal/kwh

Reduction in fuel consumption of the order of 13524 - 20286 tons per year for one 500 MW unit

Reduction in CO2 emission of the order of 16905 - 25357 tons per year for one 500 MW unit

Annual savings of Rupees 18.8 – 28.2 million for one 500 MW unit

Assumptions : PLF - 80%, GCV - 3500 kcal/kg, Coal cost - Rs. 1400 per ton

Impact of Cooling Tower Performance

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0

5

10

15

20

25

96.7 92.5 88.7 85.1 81.8 78.8 76 72.5 70 67.9 65.8 63.8 62

HR

Lo

ss

(K

ca

l/k

wh

r)

Capability (%)

CT Capability vs HR Loss (200 MW)

Seri…

6 0

5

10

15

20

25

97.4 95 91.6 88.4 85.4 82.7 80.1 77.6 75.3 73.2 71.1 69.2 67.3 65.6 64

LO

ss in

HR

(K

ca

l/kw

hr)

Capability (%)

CT Capability vs HR (500 MW)

Series1

No standard curve was available with OEM / CTI for CT Capability vs Heat rate

Typical curves were developed for 200 MW & 500 MW units to assess the heat rate loss due to poor CT performance

Impact of Cooling Tower Performance

CenPEEP Optimization of CT Performance

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Concrete Splash Bars got damaged affecting the heat transfer. Complete splash bars replacement with concrete or PVC type improved the performance.

Choking of Film type fills due to airborne or waterborne dusts or silts. Mechanical cleaning or In-situ cleaning with water improved the performance. Replacement of 17 mm flute by 19 mm flute film packs also reduced choking.

Nozzles getting choked due to balls coming from OLTC. Screens provided in hot water basin and the OLTC screens have been repaired.

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Falling of nozzles is repeated causing unequal water distribution.

Plant and wild grass growth in Cooling Tower area obstructing the air flow.

Heavy algae growth on splash bars and tower structure affecting the performance. Proper chemical treatment has improved the performance.

Reduced air flow through the tower affecting the performance. Air flow optimization done by adjusting the blade angle based on air flow or Fan power measurement.

Optimization of CT Performance

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Inefficient air path that do not pass through the fill i.e. heat transfer zone

Improper Sealing of shaft hole of fan.

Improper Sealing of door openings of fan chamber.

Improper Sealing of the fan hub area.

Increased blade tip clearances

Increased drift handled by fan due to damaged or missing drift eliminators. Replacement of drift eliminators has been done.

Mud/slime deposit in the hot water basin resulting choking/ damage of nozzles. Cleaning of hot basins has been done.

Failure of gear boxes has decreased the availability of Cooling Tower fans i.e. failure of input / output shafts, failure of bearings, failure of worm wheels etc.

Mud or ash deposits in cold water basins. Two stage screening of CW water at CT outlet and proper cleaning of cold water basin have improved the performance.

Improper Chlorination & Shock dozing to maintain required FRC

Air recirculation from one tower to other

Optimization of CT Performance

CenPEEP Liquid/Gas Ratio in Cooling Towers

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In a cooling tower, heat is transferred from water drops to the surrounding air by the transfer of sensible and latent heat.

Water Drop with Interfacial Film

Liquid/Gas Ratio (L = water; G = air), of a cooling tower is the ratio between the water and air mass flow rates.

Thermodynamically, heat removed from the water must be equal to the heat absorbed by the surrounding air

where: L/G = liquid to gas mass flow ratio (lb/lb or kg/kg) T1 = hot water temperature (0F or 0C) T2 = cold water temperature (0F or 0C) h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature

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Parameters to be Measured

Wet Bulb Temperature (WBT) at Tower inlet

Cold Water Temperature

Hot Water Temperature

CW Flow to each Tower

Fan Motor Power

Air Flow at each CT Fan

CT Thermal Capability including L/G ratio

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Acceptable Test Conditions

CW Flow rate : 90 – 110% of Design

Cooling Range : 80 – 120% of Design

Wet-Bulb Temp : Design +/- 8.50 C

Fan Motor Power : 90 – 110% of Design

Average wind velocity : < 4.5 m/s

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Grid setup for Cold Water Temp. measurement

Arrangement of three RTDs in a single pipe

Actual measurement of CW temperature at CT outlet channel

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Cooling Tower Thermal Performance Testing

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Grid setup for Cold Water Temp. measurement of single cell

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Cold water temperature is being measured before it falls on basin

Cooling Tower Thermal Performance Testing

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Fan Motor Power


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CenPEEP CW Flow Measurement

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CW Flow measurement using 3 hole pitot tube on underground CW header

CW Flow measurement using ultrasonic flow meter on riser

tubes

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Fan Motor Power


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CenPEEP Single Cell Air Flow measurement

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The Anemometer is to be tied with rod (light in weight) with length equal to at least the radius of CT fan top portion. The length of cable connected to anemometer should also be more than the radius of CT fan top portion.

The Platform for proper approach for air flow measurement (at the top of the CT fan) with wheels at the bottom for mobility is to be made

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The airflow and specific power consumption of individual cells

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Cell No Fan blade angle

(degree)

Fan Power (KW) Air Flow (t/hr) Specific power consumption

(W/(t/hr))

1 9 30.9 1407 21.96

2 9 31.1 1275 24.39

3 9 32.4 1322 24.52

4 9 32.1 1127 28.48

6 9 33.3 1204 27.65

7 9 33 1319 25.02

8 9 35.1 953 36.84

9 8.75 33.5 1320 25.38

10 9.25 32.6 988 32.98

11 9.25 33.1 1380 23.99

12 9.25 34.2 996 34.32

13 9 34.4 1064 32.34

14 9 32.8 973 33.69

15 9.25 32.5 999 32.55

16 9 33.3 1022 32.58

17 9 33.7 822 40.99

18 9 35.1 886 39.62

------ 33.12 (Ave) 19057 (Total) 29.55 (Ave)

New Fills

Sr. No Parameter Unit Design Predicted Actual Deviation/ Shortfall

1 Circulating water flow M3/Hr 24000 - 25105 - 1105

2 Hot water Temperature Deg C 42 - 37.9 -

3 Cold water Temperature Deg C 32 28.04 30.24 2.20

4 Inlet air WBT Deg C 28 - 20.43 -

9 Capability % 100 100 70.72 29.28

The Cooling Tower Thermal Capability Summery


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The average fan motor power was 33.12 KW as against the design value of 37 KW. Low fan power indicates that less air flow is through the tower

The average air flow of the cells with new fills are 1294 t/hr as compared to average air flow of the cells with old fills of 967 t/hr.

The specific power consumption for the cells with new fills are 25.17 W/(t/hr) (low system resistance) and the specific power consumption for the cells with old fills is 35.10 W/(t/hr) (high system resistance)

This confirms that fill cleaning / replacement has increased the air flow as well as reduced the specific power consumption.

Choked Fills New Fills

Non uniform water flow distribution

Improper sealing of Shaft holes/Doors


CenPEEP VFD for Cooling Tower Performance optimization

Use of VFD in CT Fan for APC reduction

Pilot project installed in CT – 1 Cell – 11 by NETRA

CT Single Cell Test carried out at Different VFD from 30 Hz to 55 Hz

Air flow measured at various speed

Impact of L/G ratio on CT performance

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VFD Application in CT Fan and VFD Panel

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CT - 1 Cell – 11 Performance at Different VFD Condition

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FIELD TEST PARAMETERS Units

TEST

VALUE

TEST

VALUE

TEST

VALUE

TEST

VALUE

TEST

VALUE

TEST

VALUE

VFD Position Hz 55 50 45 40 35 30

Shortfall in Cold Water

Temperature Deg C 0.37 0.90 1.61 2.35 3.67 4.10

Heat Rate Loss Kcal/kwh 2 4 8 12 18 21

Measured CW Flow T/hr 1402 1402 1402 1402 1402 1402

Actual KW of Fans during Test

(Average) KW 55.48 41.34 30.75 21.20 14.50 9.41

Cell Capability % 95.46 89.69 83.76 76.96 63.13 60.52

Air Flow T/hr 1637 1361 1327 1091 915 819

L/G Ratio 0.86 1.03 1.06 1.29 1.53 1.71

Reduction in Fan Power (%) at Low load and low ambient condition

Condition Condenser Back Pressure increase by 6 mmHg

Condenser Back Pressure increase by 11 mmHg

Condenser Back Pressure increase by 15 mmHg

With VFD 50 65 77

Without VFD 15 25 30

VFD for Cooling Tower Performance optimization

CenPEEP Conclusions

Capability Test to be done along with L/G ratio at appropriate period

Fills cleaning on opportunity & during OH

Air flow optimization based on L/G ratio test

Specific Power consumption of CT fan reflects the condition of the fill

Regular visit of cross-functional team for CT health assessment

Arrangement for proper temperature measurement

Proper chemical treatment

Vegetation growth to be avoided

VFD application in CT Fan for APC reduction 22

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